Composite and support structures

ABSTRACT

A method of constructing a composite structure comprises the steps of: (i) attaching light gauge sheeting to a plurality of joists that have been sufficiently closely spaced so that the sheeting is able to support a cementitious slab that has formed a composite structure with the sheeting and joists; (ii) forming the cementitious slab on the sheeting to form the composite structure; and (iii) supporting the joists until the cementitious slab has sufficiently cured.

TECHNICAL FIELD

Composite/support structures and a method of constructing such structures are disclosed. The structures find particular though not exclusive application in relation to flooring and ceilings (eg. suspended flooring and ceilings) in buildings.

BACKGROUND ART

Existing suspended floors may comprise a base layer, such as a cement slab on a ground surface, with interconnected or abutting tiles supported above the slab on struts. Existing suspended ceilings may comprise ceiling tiles suspended by a frame structure below a cement slab floor. Such suspended floors and ceilings are useful in that they provide a void space between the slab (or other corresponding floor or ceiling structure) and the tiles which may be used for feeding therethrough services, such as water, sewerage, electricity, air conditioning and communications.

To prevent excessive floor sag in existing suspended ceilings that have an underlying structural decking, the decking has been formed of heavy gauge steel.

SUMMARY OF THE DISCLOSURE

In a first aspect there is disclosed a method of constructing a composite structure comprising the steps of:

-   (i) attaching light gauge sheeting to a plurality of joists that     have been sufficiently closely spaced so that the sheeting is able     to support a cementitious slab that has formed a composite structure     with the sheeting and joists; -   (ii) forming the cementitious slab on the sheeting to form the     composite structure; and -   (iii) supporting the joists until the cementitious slab has     sufficiently cured.

Heretofore the use of light gauge sheeting in a composite structure has not been contemplated due to perceived unacceptable sag and perceived deficiencies in strength of the resultant structure. With the disclosed method it has surprisingly been discovered that a composite structure of sufficient strength can be constructed using light gauge sheeting whilst also avoiding unacceptable sag. For example, in step (iii) the joists can be supported with supports (such as temporary props) during curing of the cementitious slab to form the composite structure. These supports can then be removed once the cementitious slab has sufficiently cured.

The method enables a light gauge sheeting such as a roof or wall cladding to be used. For example, the sheeting can comprise light gauge trapezoidal coated steel sheeting of a typical thickness of just 0.5 mm. This can make the resultant composite structure very cost effective, easy and expedient to produce. The light gauge sheeting may be formed from profiled sheets that overlap at their edges when arranged on the joists.

In one form the joists can be spaced at centres that prevent the sheeting from excessive sagging during forming of the cementitious slab on the sheeting. For example, joist spacings can be employed that are around 0.75 m apart (cf. the spacing employed with known thicker [ie. thicknesses 0.75, 1.0, 1.2 mm] and more deeply ribbed steel decking having typical spacings of 2 to 3 metres between permanent or temporary supports).

In one embodiment the sheeting can be attached to the joists by a plurality of self drilling, self tapping fasteners. After attaching the sheeting to the joists each fastener can extend up into the slab formed on the sheeting to fasten the slab into the composite structure. Further, each fastener can be passed though a sleeve that, through the engagement thereof by a head of the fastener, fixes the sheeting against a respective joist during formation and curing of the slab. Such a sleeve also increases the bearing area between the fastener and the cementitious slab, improving the strength of the composite structure.

In one form each joist can be of open web construction incorporating top and bottom chords interconnected by a series of discrete web members, to define a type of elongated truss. Whilst the web members can be angled in the truss, in one form, so as to provide maximum underlying support, the web members can be aligned generally perpendicular to the top and bottom chords, and just external faces of the webs can be welded to the chords at spacings along the joist that are equal to the depth of the joist. Welding at just the external faces of the webs has been found to provide sufficient truss strength and yet minimises fabrication costs and time. Also, in a joist of this form it has again been discovered that the webs and chords may each be of rectangular hollow section of light gauge steel, making the resultant composite structure very cost effective, easy and expedient to produce. The open web construction can also allow for a provision of services within the joist space, such as electricity, water, air conditioning and/or communications.

In another form each joist can be a hollow flange beam. For example, the beam can comprise a planar elongate web that interconnects opposing torsionally rigid elongate hollow flanges. Again, in a joist of this form it has again been surprisingly discovered that the beam can be of light gauge steel (for example, taking the form of a so-called LiteSteel™ beam owned and produced by LiteSteel Technologies Pty Ltd) with the attendant benefits. The web in the beam can have openings therethrough to again allow for a provision of services within the joist space.

When attaching the sheeting to the joists the self drilling, self tapping fasteners can be inserted into the top chord of the truss or the top flange of the beam.

In addition, the ends of each joist can be fastened between opposing flanges of channel section, which can in turn be supported on spaced posts to suspend the structure. Yet again, it has again been discovered that the channel section and posts can be of light gauge steel with the attendant benefits. In one embodiment, the joist ends can be fastened to the channel section by self drilling, self tapping fasteners which extend through the channel and into the joist. This can allow the joists to provide torsional and lateral restraint to the channel section which can in turn allow for lighter gauge channel section to be employed in the structure, with the attendant benefits (eg. an inline galvanized, cold rolled channel section of typically 6 mm thickness can be employed).

The cementitious slab may incorporate steel reinforcing. The steel reinforcing may be in the form of wire mesh.

In a second aspect there is disclosed a shear member for a composite structure that comprises a plurality of joists which underlie and support a cementitious slab in the structure, the shear member being fixable to a respective joist to project upwardly therefrom such that in use it becomes partially embedded in the slab, the shear member comprising a sleeve that is located on a shank thereof whereby, when the shear member is fixed to a respective joist, the sleeve is retained on the shank by a head of the shear member.

As in the first aspect, such a sleeve can increase the bearing area between the shear member and the cementitious slab, improving the strength of the composite structure by providing a composite action between the slab and the joist. The sleeve can also allow the use of a smaller diameter shear member (eg. a self drilling, self tapping fastener of around 6 mm diameter).

In the second aspect the joists can underlie and support a decking and the cementitious slab can be located on such a decking in the structure. The shear member can then be fixed through the decking to its respective joist, with the shear member head acting on the sleeve to urge it against the decking to fix the decking against the joist. Again, the decking can be a roof or wall cladding of light gauge steel.

The shear member of the second aspect may also be used in the composite structure construction method of the first aspect.

In a third aspect there is disclosed a support structure comprising:

-   a plurality of joists; and -   at least one beam of channel section that has top and bottom flanges     interconnected by a web;

wherein the plurality of joists are supported by the at least one beam, the joists projecting from the at least one beam and having one end thereof disposed between the top and bottom flanges.

Such an arrangement again allows the at least one beam of channel section to be formed of a light gauge metal (eg. steel). This channel section can then be secured to a post (itself optionally formed of a light gauge metal such as steel) to suspend the support structure. In addition, each joist can be secured to the channel section by self drilling, self tapping fasteners.

The plurality of joists may be as defined in the first aspect (eg. open web truss or hollow flange beam). The support structure of the third aspect may also be used in the construction of the composite structure according to the first aspect.

In a fourth aspect there is disclosed a composite structure comprising:

-   a support structure according to the third aspect; and -   a cementitious slab supported on the plurality of joists.

In a fifth aspect there is disclosed a composite structure comprising:

-   a plurality of joists according to the first aspect; and -   a cementitious slab supported on the plurality of joists.

Heretofore the use as joists in the form of the open web truss or hollow flange beam of the first aspect would not have been contemplated in a composite structure, as the use of light gauge components has not previously been contemplated in suspended construction applications.

In the composite structure of the fifth aspect the joists can again underlie and support a decking, and the cementitious slab can be located on this decking.

As will be understood, spatial terms, such as “top”, “bottom” and “vertical”, used in the claims and throughout the specification are not to be limited in their interpretation in a strict literal sense, but are provided as terms of convenience. Also, the noun “building” as used in the specification is to be interpreted as including, but not being limited to, small and large scale domestic, industrial, commercial and community buildings and like structures.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the structure and its method of construction will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of a first support and composite structure embodiment;

FIG. 2 is a perspective view of several components of the composite structure illustrated in FIG. 1;

FIG. 3 is a cross-sectional detail side elevation of a support and composite structure in accordance with the embodiment illustrated in FIG. 1;

FIG. 4 is a perspective view illustrating one form of mounting a channel section to a post; and

FIG. 5 is a perspective view of an alternative joist embodiment.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Referring to FIGS. 1 to 3 a composite structure is shown in the form of a suspended floor 10. The floor 10 comprises a plurality of joists 12 each comprising a top chord 14 and a bottom chord 16 interconnected by web members 18. In this embodiment, the web members 18 are generally perpendicular to the generally parallel top and bottom chords 14, 16 providing a “ladder-like” Vierendeel truss structure. This arrangement has been found to provide maximum support to an overlying deck and concrete slab to prevent sagging. In an alternative embodiment at least some of the web members may be oblique to the generally parallel top and bottom chords 14, 16 such that the joists 12 are in a triangular truss form.

The top and bottom chords 14, 16 and web members 18 are preferably formed from rectangular hollow section (RHS) members of eg. a light gauge galvanised steel having a thickness of 2 mm. An example of a type of RHS which may be used is DURAGAL® RHS members produced by Onesteel Ltd. While not essential, for convenience, RHS members of the same lateral cross-sectional dimensions may be used to form each of the top and bottom chords 14, 16 and the web members 28. The width of the joists 12, from top chord 14 to bottom chord 16 is preferably about 300 mm, but may be in the range of 100 mm to 800 mm. Also, the RHS members which form the joists 12 preferably have cross-sectional dimensions of 65 mm×35 mm, but may be smaller or larger, depending on the application, load bearing requirements, span required, and so on.

A decking 20 overlies the top chord 14 of the joists 12 and in this embodiment comprises overlapping profiled sheets 20 a. It has surprisingly been discovered that a light gauge sheet of roof or wall cladding can be used for the decking 20. For example, the sheets can comprise light gauge trapezoidal coated steel cladding of thickness 0.5 mm. The sheets can overlap at their edges when arranged on the joists or can abut.

Commercial examples of profiled sheets 20 a that can be used include TRIMDEK® cladding manufactured by Bluescope Steel Limited and profiled cladding such as COLORBOND® manufactured by Bluescope Steel Limited. In other applications heavier gauge sheets such as BONDEK® manufactured by Bluescope Steel Limited and CONDECK HP® manufactured by Stramit Corporation Pty Ltd.

In other applications non-profiled non-metal sheets that either overlap or abut may be used. Such sheets may comprise cut timber or compressed particulate timber sheets, panels or boards, or compressed fibre sheets, such as plasterboard.

The decking 20 is fixed to the joists 12 by shear members in the form of mechanical fasteners 22. The mechanical fasteners 22 may comprise one or more of screws, roof bolts, other suitable bolts, nails, glue and so on. In this embodiment, the fasteners 22 are self-drilling, self-tapping fasteners having a diameter of 6 mm and a length of 90 mm.

Referring specifically to FIG. 3 it will be seen that each fastener 22 is used in conjunction with a sleeve in the form of an 80 mm length of 7 mm inner diameter tubing 23, where the fastener 22 is inserted into the tubing prior to screwing it into the decking 20 and joists 12. The head 22 a of the fastener acts on the tubing 23 to in turn urge the tubing against the decking 20 to fix it to the top chord 14 of joist 12. This is useful in acting as a raised shear member, as will be described in more detail below. The tubing in this embodiment is copper tubing, however may be produced from other material, such as PVC or steel as required. In a variation, only some of the fasteners employ the tubing. The tubing increases the bearing area between the fastener and the concrete slab, improving the strength of the composite structure. The tubing can also allow the use of a smaller diameter self drilling, self tapping fastener of around 6 mm diameter, which can easily pierce the light gauge components.

A cementitious slab in the form of a concrete slab 24 is supported by the decking 20. In the preferred embodiment, the slab 24 is poured and set in situ on the decking 20 to a thickness of about 100 m, although in alternative embodiments, the slab 24 may be less or more thick, preferably within a range of 50 mm to 150 mm. The concrete thickness can be adjusted with the joist spacing to achieve load sharing between the joists. Steel reinforcing in the form of relatively lightweight reinforcing mesh 25 can be used to reinforce the slab 24 in tension. In some applications a relatively heavier steel reinforcing can be employed such as used when laying a concrete ground slab, however the lighter weight reinforcing mesh is preferred.

Because each of the fasteners 22 have a portion which extends about 80 mm from the decking 20, the extended portions have the effect of extending the shear members into the slab 24 which is poured and set thereupon, thus helping to secure and prevent lateral, or shear, movement of the slab 24 on the decking 20. The length of the tubing is determined in relation to the thickness of the slab 24. It is preferred that at least 20 mm of the slab overlies the extended portion of the fastener, for example to prevent rust of the fastener through the portion of slab thereover and to prevent cracking of the slab 24. Alternatively, for the 100 mm slab thickness, a shorter length of tubing may be used, such that the fastener does not extend as far into the slab. Alternatively, when slabs of less thickness are employed, tubing of less length may be employed to ensure at least 20 mm cover of slab over the fasteners. In further alternatives, tubing of different inner diameter, for example in the range of 4 mm to 12 mm, may be used, and fasteners of different length and diameter may be used, provided the fastener can be inserted into the tubing and that the head 22 a of the fastener is greater than the inner diameter of the tubing. The length of fastener is determined depending on the length of the tubing, thickness of decking 20 and thickness of a top wall of the joist.

During pouring and setting of the slab 24, the joists 12 may be supported from below proximate a general centre thereof (approximately mid-span) until the concrete is cured to an acceptable degree (between 5 and 25 days), to resist unwanted vertical deflection of the joists 12 which may otherwise occur due to the liquid or relatively flexible nature of unset concrete, until the concrete slab 24 is cured. The mid-span support can then be removed. Temporary struts are typically employed.

Using profiled sheeting as the decking 20 base for the slab 12 may have several advantages. Roof and wall cladding are readily available, are easily transportable, are cost effective, and techniques used in roofing and cladding can be adapted for fixing the sheets 20 a to the joists 12. They provide a reliable base for laying of the slab 24 thereon and their profiled configuration helps prevent lateral movement of the slab 24 on the decking in directions oblique or perpendicular to the elongate direction of the profiling of the decking. The extended portions of the fasteners 22 can resist lateral movement of the slab 24 on the profiled decking 20 in the direction of the profiling.

A ceiling may be attached to an underside 26 of the bottom chord 16. The ceiling may be any conventional ceiling type, such as precast plaster panels, plaster board, and so on. The open structure of the joists provides a convenient void which can be used to place services, such as water supply, sewerage, electricity, air-conditioning, communications cables etc.

Referring again to FIG. 1, there is illustrated the composite structure in use in a support structure 28. The support structure comprises beams in the form of C-section (channel section) 30 attached to and supported by posts 32 (as shown in FIG. 4). The posts may optionally be formed from SHS members of eg. light gauge galvanised steel of 2 mm thickness and 90 mm×90 mm cross-section. The posts 32 are typically secured to a base slab or pad via feet 32 a or secured using other known means.

The C-section 30 comprises top and bottom flanges 34, 36 interconnected by a web 38. At least some of the C-section 30 is configured such that the “C” faces inwardly of the structure 28. In this manner, in use, the ends 40 of the joists 12 can be supported by at least the bottom flange 36 of a respective C-section 30, where the ends 40 of the joists 12 are disposed between the top and bottom flanges 34, 36 of a respective C-section 30. In this embodiment, each end 40 of the joists 12 comprises a web member 18 extending between the ends of the top and bottom chords 14, 16, such that the ends of the top and bottom chords 14, 16 do not extend beyond the web member 18 therebetween. In this manner, the joists 12 can be secured to their respective C-section 30 by fasteners which extend through the web 38 and into respective web members 18 at the ends 40 of the joists 12. Connecting the joist ends to C-section 30 allows the joist to provide lateral and torsional restraint to the C-section.

The internal distance between the top and bottom flanges 34, 36 can be selected to be greater than the combined height of the joists 12 with decking 20 thereupon, such that the flanges 34, 36 of the C-section beam 30 can accommodate the joists 12 and decking 20 therewithin. A right angle bracket 41 can also affixed to the top flange 34 of the C-section 30 to support the end 42 of the slab 24. Alternatively, the distance between the flanges 34, 36 may be sized to snugly accommodate the joists 12 only, or even the composite structure: joists 12, decking 20 and slab 24.

The joists 12 are spaced at regular, standard intervals, preferably 750 mm centres, but may lie in the range of 300 mm centres to 900 mm centres. RHS member joists 12 in the configuration described above having top and bottom chords 14, 16 connected by web members 18 have been shown to have acceptable vertical deflection resistance under load.

Referring to FIG. 4, there is illustrated two C-sections 30 attached to respective opposing sides of a post 32. Connected (via bolts, nuts and washers 43) so as to extend between the post and each top flange 34 and bottom flange 36 of each C-section is a respective 90 mm long angle 44 of dimension 75 mm×75 mm×6 mm thickness. Connected to extend between the post and each web 38 of each C-section is a respective 190 mm long angle 46 of dimension 75 mm×75 mm×6 mm thickness. These fittings allows load transfer from the C-section on one side of the post to the C-section on the other side of the post, again reducing the size of the C-section required.

Referring to FIG. 5, there is illustrated an alternative joist in the form of a hollow flange beam 50. The beam comprises a planar elongate web 52 that interconnects opposing torsionally rigid elongate hollow flanges 53, 54. Again, the beam can be of light gauge steel. In an example a so-called LiteSteel™ beam owned and produced by LiteSteel Technologies Pty Ltd can be employed. The web 52 can have openings or apertures formed therethrough to again allow for a provision of services within the joist space. Again, the self drilling, self tapping fasteners that connect to the slab can be inserted into an in-use top hollow flange of the beam 50, and self drilling, self tapping fasteners that connect the joist to the C-section 30 can extend up through the flange 36 to be inserted into an in-use bottom hollow flange of the beam 50.

Construction Example

A specific method of constructing a composite concrete deck suspended flooring system using thin section steel joist, beam and post will now be described.

Steel roof sheeting of 0.47 mm thick was attached to the top of the joists. The joists were spaced at centres that prevented the roof sheeting from excessive sagging during placement of the concrete with temporary supports located under the middle of certain joists. The steel sheeting was attached to the joists by the use of self drilling, self tapping screws, which were 6 mm in diameter. The screws were passed though a steel sleeve. The sleeve held the steel decking against the joists during the placement and curing of the concrete. The screws were inserted in the hollow section top chord or hollow flange of the joist, which was typically 2 mm thick.

Concrete was poured onto the sheeting to cover the self drilling, self tapping screws. The steel tubular sleeves were observed to increase the bearing area between the screw and the concrete.

In one application the joists employed were Vierendeel trusses. The vertical webs were attached to the chords by welding the external faces of the webs (not the internal faces) to the chords. The reduced amount of welding minimised fabrication costs. The webs were of the same material as the chords, and were placed at spacings along the joist equal to the depth of the joist. In use, the opening between the webs allowed services (plumbing, drainage, electrical, communication, air-condition, etc) to be run through the joists. The joists were fabricated from galvanized rectangular steel hollow sections typical 65 mm wide, 35 mm deep and 2.0 mm thick.

The joists were inserted between the flanges of inline galvanized, cold rolled channel section (C-section) support beams. The support beams were 6 mm thick. This thickness allowed the joists to be fastened to the channel with self drilling, self tapping screws. This fastening method allowed the joists to provide torsional and lateral restraint to the beams.

The support beams were attached to posts, which were galvanized coated steel hollow sections, typically 90×90 in section dimensions and 2 mm thick. Proprietary fittings were used to attach the beams to the posts. The fittings had internal sleeves that fitted into the post and angle cleats for attaching the top and bottom flanges and the web of the beam to the column. The fittings allowed load transfer from a beam on one side of the post to a beam on the other side of the post, reducing the size of the beam required.

The resultant structure produced a floor of stiffness that allowed the attachment of a ceiling to the underside of the joists. The deflection of the floor was observed to be minimal when the floor was worked on above, thereby preventing movement of light fittings, ceiling fans, etc attached to its underside. The resultant structure made use of the following interactions:

-   adjustment of the joist spacing and concrete thickness to achieve     load sharing between the joists; -   use of steel tubular sleeves over the self drilling, self tapping     screws to provide a composite action between the concrete deck and     the thin section steel joist; -   the joist connection to the beam allowing the joist to provide     lateral and torsional restraint to the beam; and -   the beam to post connection providing load transfer from a beam on     one side of the post to a beam on the other side of the post.

While the composite structure of the above description has been provided in respect of a suspended floor, it will be understood that the structure could be in other forms, such as a suspended ceiling, a roof structure, a balcony, and so on.

The cementitious slab may comprise, for example, a fibre concrete slab or a lightweight cementitious slab, such as lightweight slabs comprising foam beading. In other forms, the slab may be preformed in sub-slabs or in predetermined shapes for on site installation. Where pre-formed slabs are used, no decking may be required.

While the composite structure has been described in reference to specific embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes may be made to the composite structure without departing from its scope.

In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” and variations such as “comprises” or “comprising” are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments. 

1. A method of constructing a composite structure comprising the steps of: (i) attaching light gauge sheeting to a plurality of joists that have been sufficiently closely spaced so that the sheeting is able to support a cementitious slab that has formed a composite structure with the sheeting and joists; (ii) forming the cementitious slab on the sheeting to form the composite structure; and (iii) supporting the joists until the cementitious slab has sufficiently cured.
 2. A method as claimed in claim 1 wherein the sheeting is attached to the joists by a plurality of self drilling, self tapping fasteners.
 3. A method as claimed in claim 2 wherein, after attaching the sheeting to the joists, each fastener extends up into the slab formed on the sheeting.
 4. A method as claimed in claim 2 wherein each fastener is passed though a sleeve that, through engagement thereof by a head of the fastener, fixes the sheeting against a respective joist during formation and curing of the slab.
 5. A method as claimed in claim 1 wherein each joist is of an open web construction incorporating top and bottom chords interconnected by web members.
 6. A method as claimed in claim 5 wherein the web members are aligned generally perpendicular to the top and bottom chords, and external faces of the web members are welded to the chords at spacings along the joist that are equal to a depth of the joist.
 7. A method as claimed in claim 1 wherein each joist is a beam that comprises a planar elongate web that interconnects opposing torsionally rigid elongate hollow flanges.
 8. A method as claimed in claim 1 wherein components of each joist and the sheeting are each formed of light gauge steel.
 9. A method as claimed in claim 5 wherein the sheeting is attached to the joists by a plurality of self drilling, self tapping fasteners, and the fasteners are inserted into a top chord.
 10. A method as claimed in claim 1 wherein ends of each joist are fastened between opposing flanges of a channel section.
 11. A method as claimed in claim 10 wherein the joist ends are fastened to the channel section by self drilling, self tapping fasteners which extend through the channel and into the joist ends.
 12. A method as claimed in claim 1 wherein in step (iii) the joists are supported with supports during curing of the cementitious slab to form the composite structure, which supports are removed once the cementitious slab has sufficiently cured.
 13. A shear member for a composite structure that comprises a plurality of joists which underlie and support a cementitious slab in the structure, the shear member being fixable to a respective joist to project upwardly therefrom such that in use the shear member becomes partially embedded in the slab, the shear member comprising a sleeve that is located on a shank of the member whereby, when the shear member is fixed to a respective joist, the sleeve is retained on the shank by a head of the shear member, the sleeve being connected to reinforcing which in use is embedded in the cementitious slab.
 14. A member as claimed in claim 13 wherein the joists underlie and support a decking and the cementitious slab is located on the decking in the structure, the shear member being fixable through the decking to its respective joist, with the shear member head acting on the sleeve to urge it against the decking to fix the decking against the joist.
 15. A member as claimed in claim 14 wherein the decking is a light gauge steel roof or wall cladding.
 16. A member as claimed in claim 13 that is in the form of a self drilling, self tapping fastener.
 17. A member as claimed in claim 13 that is adapted for use with a composite structure having light gauge sheeting attached to the plurality of joists.
 18. A support structure comprising: a plurality of joists, each joist comprising either: a top chord and a bottom chord interconnected by web members in an open web construction; or a beam that comprises a planar elongate web interconnecting opposing torsionally rigid elongate hollow flanges; and at least one channel section that has a top flange and a bottom flanges interconnected by a web; wherein the plurality of joists are supported by the at least one channel section, the joists projecting from the at least one channel section and having one end thereof disposed between the top flange the bottom flanges, whereby the top chord or one of the hollow flanges is secured to the top flange, and the bottom chord or an other of the hollow flanges is secured to the bottom flange, respectively.
 19. A support structure as claimed in claim 18 wherein the at least one channel section comprises light gauge steel and the joists are secured to the channel section by self drilling, self tapping fasteners.
 20. A support structure as claimed in claim 18 wherein, in the joists of open web construction, the web members are aligned generally perpendicular to the top and bottom chords, and external faces of the web members are welded to the chords at spacings along the joist that are equal to a depth of the joist.
 21. A composite structure comprising: a support structure according to claim 18; and a cementitious slab supported on the plurality of joists.
 22. A composite structure comprising: a plurality of joists; and a cementitious slab, formed, cured and supported on the plurality of joists.
 23. A structure as claimed in claim 22 wherein the joists underlie and support a decking and the cementitious slab is located on the decking.
 24. A method as claimed in claim 7 wherein the sheeting is attached to the joists by a plurality of self drilling, self tapping fasteners, and the fasteners are inserted into a top hollow flange of each joist. 